Because the craft foam remains flexible, in order to hide a seam, you need to use a flexible product as well. Anything rigid you use will pull away upon the first flex, creating a crack, which is no better than a seam.
I've never tried this, but you might want to consider something like bathroom caulk. They are designed to fill cracks, and remain flexible, plus they have a long working time, so you can work it into the crack with out it showing. Stay away from silicone caulk; paint will not stick to it.